Previously vibration forces used in pile driving have been applied usually in a longitudinal direction, but sometimes laterally to assist penetration under the weight of the pile when a steady force is applied, and also alternating with the hammer blows. Prior proposals involve the use of mechanical vibration, induced by out-of-balance weights with some provision for harmonics but not truly a variable frequency. The frequency used is based on pile resonance which is a relatively low frequency. There are other factors involved that affect optimum frequency such as particle size of soil to be penetrated and degree of compaction and it is desirable that the applied vibration should have a variable frequency capability.
A variable vibrator on the basis of a closed loop electrohydraulic control valve has been developed for pile driving and applied with some degree of success to dry and sea bed pile driving, using alternately vibration and hammer driving.
The limitation of this system is that it can produce a maximum frequency of 100-150 Hz and requires a very high powered hydraulic pumping system having a capacity in the range of 250 horsepower to possible over 1000 horsepower for North Sea oil applications. Moreover, there is a tendency for the particles to become compacted during the vibration phase, thus reducing the penetration rate during the hammer phase.